yanmar 2gm20 - freewheel prop or not?

coopec said:
Yanmar gear position

https://www.google.com.au/#q=yanmar+locked+spinning+propeller
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The first link from google gives:
Does a propeller cause more drag freewheeling or stationary?

Consider riding in a HELICOPTER high in the sky. Ooops, the turbine engine runs out of fuel.~Now, we are in trouble!~If we lock the propeller blades from turning we will have an awful day because we will not get any drag (or in this case, lift) from the blades.~If we auto rotate (allow the blades to spin) using our altitude and speed of descent to rotate the blades we will be able to land safely. How is this possible?

I think a spinning prop on a yanmar is not exactly like a helicopter, neither is it exactly like an outboard with very little resistance to the prop turning....
 
rotrax said:
Having read his book a good number of times I think the word "Raced" is perhaps not what he actually did-especially compared to how they go about it today.

Sir Robin sailed his boat around the world on his own and, knowing he had the slowest boat of any of his potential rivals, he sort of kept his fingers crossed he would get home first-which he did.

Raced-no not really........................
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Yes, his attitude to the whole affair seems wonderfully refreshing when the whole ethos of competitive 'sailing' seems to have be obscured by technology and sponsorship pressures. Of course, he did want to win - just in his own rather laid back way.
I would love to know if removing the prop is something that he considered and discounted. I imagine he must have. Even a fraction of a knot must be significant when you are circumnavigating, so I assume that he considered the engine as an essential piece of safety equipment - even though he never quite got around to fixing it!
 
lw395

Are you saying that some of the gravitational pull on the helicopter is used to rotate to blades and the rest is used up in air resistance and pull on the helicopter. So the helicopter falls more slowly with the blades turning. I've read a spinning prop turning an alternator will slow the yacht by a knot e.g 5 knots instead of 6
 
coopec said:
lw395

Are you saying that some of the gravitational pull on the helicopter is used to rotate to blades and the rest is used up in air resistance and pull on the helicopter. So the helicopter falls more slowly with the blades turning. I've read a spinning prop turning an alternator will slow the yacht by a knot e.g 5 knots instead of 6
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A helicopter definitely falls more slowly with the blades rotating, and still retains a modicum of control. If the blades stop completely it would drop like a stone. Earlier I described a very primitive experiment using a childs "windmill" toy which demonstrates this well (though not, apparently, why you should lock the prop on your boat)

EDIT: If the gearbox is in Neutral, the alternator wouldn't be part of the equation, would it?
 
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lw395 said:
The first link from google gives:
Does a propeller cause more drag freewheeling or stationary?

Consider riding in a HELICOPTER high in the sky. Ooops, the turbine engine runs out of fuel.~Now, we are in trouble!~If we lock the propeller blades from turning we will have an awful day because we will not get any drag (or in this case, lift) from the blades.~If we auto rotate (allow the blades to spin) using our altitude and speed of descent to rotate the blades we will be able to land safely. How is this possible?

I think a spinning prop on a yanmar is not exactly like a helicopter, neither is it exactly like an outboard with very little resistance to the prop turning....
Click to expand...

The key is the forward motion of the helicopter producing lift from the rotor in the same way as an autogyro. If you let it descend vertically you won't make a safe landing.
I think a more valid comparison is a light aircraft or motor glider. If you stop the prop, there is considerable drag from the stalled blades, but it is less than the work that a rotating (and unstalled) propeller does in turning the engine to which it is rigidly coupled, so you are better off with the prop stopped. (You raise the nose and drop the speed until the prop stops, then put the nose down and regain normal speed. The prop stays stopped unless you stick the nose down and fly way above normal speed in which case you might get it to turn again, rotating the engine.)
The difference with a yacht is that you can put the drive in neutral and then the work extracted from the boat's motion to turn just the prop and the low friction of the gearbox output shaft is less than the work extracted to drag a stationary stalled prop through the water. Compared with the plane, you have to make an effort to stop the prop, and if you release it, it will usually rotate again even at low speed because there is so little resistance.
If the prop was rigidly coupled to the engine, and you could sail fast enough for it to rotate the engine, then no doubt just as in the case of the plane, it would be more efficient to lock the prop and accept the drag of the stalled blades.
 
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Plevier said:
The key is the forward motion of the helicopter producing lift from the rotor in the same way as an autogyro. If you let it descend vertically you won't make a safe landing.
I think a more valid comparison is a light aircraft or motor glider. If you stop the prop, there is considerable drag from the stalled blades, but it is less than the work that a rotating (and unstalled) propeller does in turning the engine to which it is rigidly coupled.
The difference with a yacht is that you can put the drive in neutral and then the work extracted from the boat's motion to turn just the prop and the low friction of the gearbox output shaft is less than the work extracted to drag a stationary stalled prop through the water.
If the prop was rigidly coupled to the engine, and you could sail fast enough for it to rotate the engine, then no doubt just as in the case of the plane, it would be more efficient to lock the prop.
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It seems to me everybody has avoided measuring the actual torque needed to turn the gearbox.
On an outboard, it's very very low, you can do it with your little finger.
On a typical propshaft and cutless bearing, it seems to be significant.
Are saildrives closer to outboards?

I would guess whether we are talking about a 3 blade prop with high blade area or a prop with two narrow blades will also make a difference.
Narrow aerofloils will stall more dramatically and go into a lower drag mode.

Whatever, just get a folding prop.
Then you can feel the increase in speed when you stop it churning by selecting astern, forcing it to fold.
 
lw395 said:
It seems to me everybody has avoided measuring the actual torque needed to turn the gearbox.
On an outboard, it's very very low, you can do it with your little finger.
On a typical propshaft and cutless bearing, it seems to be significant.
Click to expand...

The torque on the gearboxes of both my boats is insignificant. I took the shaft out of the motorsailer last week, it has a BMC 2.2 diesel and a fairly large Hurth gearbox. Turning the shaft showed resistance to be extremely low, even with the oil in the box at 10C or less. In the water it has a big three-bladed prop, which I doubt would even notice the tiny drag of the gearbox.

With the Yanmar I have not turned the shaft with the prop detached for a long time, but with the prop attached the torque is very low indeed.
 
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